A case study of floor failure characteristics under fully mechanised caving mining conditions in extra-thick coal seams.

To determine the failure characteristics and evolution regularity of the floor strata during fully mechanised top-coal cave mining in typical deep and extra-thick seams in western China, the 61303 working face of an Ordos mine was selected as the engineering research object. A comprehensive monitoring method combining a BOTDR (Brillouin Optical Time-Domain Reflection) distributed fiber strain test and a borehole resistivity CT (Computerised Tomography) test was adopted. The results show that floor stress distribution of the deep-buried and extra-thick coal seam is significantly affected by the different depths of rock lithology. At the interface of the rock strata with a large difference in the elastic modulus, phenomena such as the asynchronism of strata movement and obvious differences in failure easily occur. The failure depth of the floor strata in the 61303 working face is approximately 15.90 m, and the influence depth of the floor disturbance is approximately 32.70 m. Under the influence of the mining pressure, floor stress distribution and crack evolution have obvious spatial and temporal effects. In different inclined boreholes, the data captured by the cable have different values and the fracture locations of the cable also differ. Compared with a single borehole, multiple boreholes with different inclinations, directions and locations can provide more comprehensive and reliable data trends. The knowledge obtained by this monitoring can provide reference information for the study of floor damage under similar conditions and the formulation of technical measures such as those that prevent mine water disasters. [ABSTRACT FROM AUTHOR]